Anti-islanding detection scheme for distributed power generation

Abstract

An arrangement is provided for detecting the occurrence of an islanding condition in the connection of a distributed power generation source to an electrical power system or utility. In accordance with the present invention, a voltage or current signal is injected into the system, and the resulting system impedance is determined. The resulting determination is used as an indicator of the islanding condition. The detection arrangement allows the detection of islanding conditions even when the load is well matched to the generated power.

What is claimed is:  
     
       1. A method of detecting an open circuit condition in an electrical system for power supply from a central generator and distributed power generator to a load, the method comprising: 
       injecting a signal into the system;  
       determining a system impedance in response to the injected signal;  
       comparing the determined system impedance with a predetermined threshold; and  
       identifying an occurrence of an open circuit or abnormal condition based on the comparison.  
     
     
       2. The method of  claim 1 , wherein the signal is injected on one phase of the supplied power. 
     
     
       3. The method of  claim 1 , wherein the signal is injected on multiple phases of the supplied power. 
     
     
       4. The method of  claim 2  wherein the signal is injected on each phase of the supplied power with a different phase angle for each phase. 
     
     
       5. The method of  claim 1  wherein injecting a signal comprises injecting a fundamental power-frequency voltage to produce a real power flow. 
     
     
       6. The method of  claim 5  wherein the injected signal comprises harmonics of the fundamental power-frequency. 
     
     
       7. The method of  claim 1 , wherein the injected signal comprises harmonics not harmonically related to the fundamental power frequency. 
     
     
       8. The method of  claim 1  wherein injecting a signal comprises injecting a fundamental power-frequency voltage to produce a reactive power flow. 
     
     
       9. The method of  claim 1  wherein comparing the determined impedance comprises comparing an impedance magnitude with a threshold value. 
     
     
       10. The method of  claim 1  wherein comparing the determined impedance comprises comparing an impedance angle with a threshold value. 
     
     
       11. The method of  claim 1  wherein the signal is intermittently injected. 
     
     
       12. The method of  claim 1  wherein the signal is injected at a point relative to a signal waveform to minimize interference, distortion, or saturation. 
     
     
       13. A system for detecting an open circuit condition in an electrical power supply system comprising: 
       a central power generator arranged to generate electrical power;  
       at least one distributed power generator arranged to operate as a local source of electrical power for a localized load;  
       a feeder network coupled to the central power generator and the at least one distributed power generator for providing electrical power to the at least one distributed power generator;  
       a signal injector connected to the power supply system and arranged to inject a signal at each phase of the generated power;  
       an impedance calculator connected to the power supply system and arranged to determine the system impedance resulting from the injected signal;  
       a comparator arranged to compare the determined system impedance with a predetermined threshold; and  
       a controller arranged to identify the occurrence of an islanding condition based on the comparison made by the comparator.  
     
     
       14. The system of  claim 13  wherein the signal injector is arranged to inject a signal comprising a fundamental power-frequency voltage to produce a real power flow. 
     
     
       15. The system of  claim 13  wherein the signal injector is arranged to inject the signal with different phase angles for each phase of the generated power. 
     
     
       16. The system of  claim 13  wherein the signal injector is arranged to inject a signal comprising a fundamental power-frequency voltage to produce a reactive power flow. 
     
     
       17. The system of  claim 13  wherein the comparator is arranged to compare the calculated impedance magnitude with a threshold value. 
     
     
       18. The system of  claim 13  wherein the comparator is arranged to compare the calculated impedance angle with a threshold value. 
     
     
       19. The system of  claim 13  wherein the controller is responsive to the comparator to disconnect the at least one distributed power generator from the feeder network if the comparison is indicative of an impedance greater than the threshold.